ci: use config.vm for kernel config when present

Recent kernel commit [0] changed selftests config snippets structure
by extracting VM specific options to the file 'config.vm'. This file
has to be used in .github/actions/vmtest/action.yml at step
'Prepare to build BPF selftests', otherwise drivers necessary for e.g.
root file system access are not compiled into the kernel, leading to
CI failures like [1].

[0] b0cf0dcde8ca ("selftests/bpf: Consolidate VIRTIO/9P configs in config.vm file")
[1] https://github.com/libbpf/libbpf/actions/runs/6830439839/job/18578379328?pr=747

Signed-off-by: Eduard Zingerman <eddyz87@gmail.com>

.github/actions/vmtest/action.yml

@@ -46,6 +46,8 @@ runs:
         cd .kernel
         cat tools/testing/selftests/bpf/config \
             tools/testing/selftests/bpf/config.${{ inputs.arch }} > .config
+        # this file might or mihgt not exist depending on kernel version
+        cat tools/testing/selftests/bpf/config.vm >> .config || :
         make olddefconfig && make prepare
         cd -
         foldable end